Micro electro-mechanical system (MEMS) and solid-state relay (SSR) devices are used as alternatives for conventional electromechanical switching devices. As is well known, the conventional devices possess some highly desirable characteristics such as low contact resistance, high voltage breakdown, and relatively high current handling capability, which characteristics make them ideal for use in telecommunication systems. However, such conventional devices are not well suited for miniaturization or integration.
MEMS and SSR devices can perform the standard functions of conventional relays and are well suited for miniaturization and integration. MEMS devices are basically miniaturized electro-mechanical devices that are fabricated using techniques similar to those used for semiconductor integrated circuits and are well suited for low cost and high volume production. MEMS device applications have been used as pressure sensors, chemical sensors, light reflectors, switches, and relays. MEMS devices are low cost devices due to the use of microelectronic fabrication techniques, and new functionality may also be provided because they are much smaller than conventional devices.
However, MEMS and SSR devices have several major shortcomings and disadvantages. The most notable disadvantage is that these devices are relatively fragile in current carrying and voltage breakdown capabilities. For example, because MEMS and SSR devices are relatively fragile, lightning or AC power surges can completely destroy them. Lightning is characterized by very high voltage and current of very short duration pulses, i.e., less than 1.0 ms, whereas AC power surges or faults are characterized by very high voltage and current of relatively long duration pulses, i.e., seconds. As a result, systems having MEMS or SSR devices therein can become disabled and/or destroyed quite easily.
There are currently different systems and methods for protecting MEMS and SSR devices from lightning and/or AC power surges. But, none of these conventional systems and methods is directed towards protecting MEMS and SSR devices that are implemented within units such as cross connect systems, e.g., the “CX 100 CrossConnect System” from Turnstone Systems, Inc. The CX100 Copper CrossConnect System is a platform that automates the physical layer infrastructure in the central office, enabling ILECs (incumbent local exchange carrier) and CLECs (competitive local exchange carrier) to remotely control, test, and manage a copper loop. Additional information regarding Turnstone System's CX100 Copper CrossConnect System can be found at its web site turnstone.com. It is also noted that other systems and units providing similar functionalities as the CX 100 Copper CrossConnect System can be implemented in the present invention.
In cross connect applications, the system can be configured in either a “single end exposed system” or a “double end exposed system.” For a more comprehensive understanding of the above-identified systems and the present invention, the following terms have been defined as follows:
(1) a “pass-through system” is a system that provides connection between an input port and an output port through a pair of metallic conductors characterized by relatively low ohmic resistance;
(2) an “ingress port” is a signal entering an equipment; and
(3) an “egress port” is a signal exiting an equipment.
FIG. 1 illustrates a simplified block diagram of a conventional single end exposed system. The single end exposed system is a system where one port, such as the ingress port 2, is connected to an “outside plant” equipment, and the other port, such as the egress port 4, is connected to an “in-building” equipment or termination unit, such as a Central Office (CO) equipment. In this diagram, the ingress port 2 is identified by terminals T1 and R1, while the egress port 4 is identified by terminals T2 and R2.
In greater detail, the ingress port 2 provides connection to the “outside plant”, an over voltage protector (“OVP”) 6, and an Equipment Under Protection (“EUP”) 8. The egress port 4 provides connection to the EUP 8 and the termination within the “in-building” equipment. The EUP 8 represents a metallic cross connect unit or the like and is implemented with MEMS or SSR devices. As discussed above, over voltage and/or over current can easily damage the MEMS or SSR devices within the EUP 8. Typically, the OVP 6 protects the MEMS or SSR devices from over voltage conditions. An over current protector (“OCP”) (not shown) can also be used to protect the MEMS or SSR devices from over current conditions. The OCP function is usually performed by the termination unit with a current limiter, such as a resistor of appropriate value.
The OVP 6 is implemented only in between the ingress port 2 and the EUP 8. Since the connection between the egress port 4 and the EUP 8 is generally not exposed to voltage surges, another OVP is not required in between the egress port 4 and the EUP 8. A co-pending U.S. patent application Ser. No. 09/677,483, commonly owned by the assignee of record, discloses improved methods and systems for protecting MEMS and SSR devices in the single end exposed system.
FIG. 2 illustrates a simplified block diagram of a conventional double end exposed system. The double end exposed system is a system where both ports, ingress 2 and egress 4, are connected to the “outside plant” equipment. The double end exposed system includes over voltage protectors, OVP-I 12 and OVP-E 14, near the ingress and egress ports, respectively, which ports can be exposed to lightning and AC power surges. In other words, lightning and AC power surges can enter from either side of the EUP 8 and thus, both sides of the EUP 8 need to be protected.
FIG. 3 illustrates a more detailed diagram of the conventional double end exposed system of FIG. 2. In FIG. 3, the term “OP” replaces the term “OVP” of FIG. 2 for ease of explanation. As illustrated, the ingress port represented by terminals T1 and R1 can be connected to the Vs and Rs, where Vs represents a surge source generator and Rs represents the corresponding source resistance. Likewise, the egress port represented by terminals T2 and R2 can be connected to a termination equipment 20. The over voltage protectors are represented by OP1 and OP2 in proximity to the ingress port, and by OP3 and OP4 in proximity to the egress port. In addition, the resistors RC1 and RC2 represent the finite contact resistance associated with the MEMS device 22. The EUP 8 is represented by the MEMS device 22 for ease of explanation.
The over voltage protectors or OPs are characterized by many parameters, but the key parameters for the purposes of understanding the present invention is the break-over or switching voltage represented by Vbo and the device on-state voltage represented by Von. Other key parameters include the current handling capability, switching speed, and the standoff voltage Vdrm. The standoff voltage Vdrm is defined as the maximum voltage across the device without having to turn the device “on,” and the break-over voltage Vbo is defined as the minimum voltage across the device to turn it on (i.e., device changing from “off” to “on” state). The selection of this voltage is dictated by the maximum voltage that the MEMS device 22 can withstand without failure. The on-state voltage Von is the voltage drop across the device when it is turned on and is generally in the range of 1.0 to 3.0 volts, depending on the amount of current flowing through the device.
For typical CO application, the required standoff voltage Vdrm is approximately 200V minimum. The break-over voltage Vbo is selected to be approximately 300V maximum, which can “fire” (break-over/switchover) to turn itself on. In fact, the device can turn itself on anywhere between 220V to 300V. This wide break-over voltage Vbo range is dictated by technology and manufacturing tolerances.
A closer view of the diagram of FIG. 3 reveals that the circuit is symmetrical with respect to the ground G. In an effort to simplify the explanation of the problem associated with this system, FIG. 4 illustrates a section (upper half) of the conventional double end exposed system of FIG. 3. FIG. 4 illustrates the terminals T1 and T2 in parallel to the ground G. As illustrated, the surge source Vs is positioned in between the terminal T1 and the ground G through its source resistance Rs. The resistor RC1 is the equivalent MEMS contact resistance, and the over voltage protectors OP1 and OP3 are assumed to include the same characteristics, nominally. Due to inherent tolerance associated with the break-over voltage Vbo of both the over voltage protectors OP1 and OP3, the following scenarios can exist.
First, when the surge source Vs is less than the break-over voltage Vbo of either of the over voltage protectors OP1 or OP3, no currents are flowing within the circuit. In other words, this means that currents i0=i1=i3=0, which means that there are no currents flowing. Accordingly, the MEMS device 22 is protected.
Second, when the break-over voltage Vbo of the over voltage protector OP1 is less than the break-over voltage Vbo of the over voltage protector OP3, the over voltage protector OP1 will turn on when surge source Vs reaches the break-over voltage Vbo of the over voltage protector OP1. Then, the surge current i0 will naturally flow through the over voltage protector OP1. This current will be essentially equal to all surge currents (i.e., i0=i1). The voltage across the over voltage protector OP1, as represented by V1, is Von in the range of 1.0 to 3.0 volts. In this scenario, there is no current flowing through the over voltage protector OP3 (i.e., i3=0) and, hence the MEMS device 22 is protected from surge currents.
Third, when the break-over voltage Vbo of the over voltage protector OP1 is greater than the break-over voltage Vbo of the over voltage protector OP3, the over voltage protector OP3 will turn on when the surge source Vs reaches the break-over voltage Vbo of the over voltage protector OP3. The voltage across the over voltage protector OP3, as represented by voltage V3, is Von in the range of 1.0 to 3.0 volts. The over voltage protector OP1 is kept in the off-state mode until the surge current flowing through the resistor RC1 and the over voltage protector OP3 produces voltage large enough to reach the break-over voltage Vbo of the over voltage protector OP1. In other words, the over voltage protector OP1 is kept in the off-state mode, and there is no current flowing through it (i.e., i1=0). In the meantime, the current flowing through the resistor RC1 and the over voltage protector OP3 is the same as the surge current (i.e., i3=i0), and such current will likely damage or destroy the MEMS device 22.
Fourth, when the break-over voltage Vbo of the over voltage protectors OP1 and OP3 are exactly the same, which is a rare case, both the over voltage protectors OP1 and OP2 will turn on and the surge current will flow through both protectors. The amount of current flowing through the two protectors will be split between the two, the amount depending on the exact impedance of the circuit. Such current through the resistor RC1 will likely damage or destroy the MEMS device 22.
As detailed above, the conventional systems and methods for protecting MEMS and SSR devices in double end exposed systems are found to be inadequate and unworkable. Accordingly, there is a need for more reliable and efficient systems and methods for protecting MEMS and SSR devices in double end exposed systems due to lightning exposure or electrical power surges.